Accumulations of static electrical charge can cause a variety of adverse effects. Discharges of static electricity are discomforting to people and can disrupt the operation of electronic equipment such as computers. Problems with static charge build-up have become particularly acute in certain industrial operations of which the manufacture of miniaturized solid state electronic components is a prominent example.
Discharges of static electricity can destroy the minute conductive paths in microchip wafers or the like. Charge accumulations on such wafers or the like also attract particulate contaminants which can cause the product to become defective.
Maintaining a high level of air ionization in the vicinity of objects which are to be protected is a highly effective technique for suppressing static charge build-up in clean rooms where electronic components are manufactured or at other locations. Charge accumulations on objects attract air ions of opposite polarity which then neutralize the charge.
Most air ionizing systems have one or more sharply pointed electrodes to which high voltage is applied. The resulting intense electrical field near the point of the electrode dissociates molecules of the constituent gases of air into positively and negatively charged ions. Ions having a polarity or charge opposite to that of the electrode are attracted to the electrode and neutralized. Ions of similar polarity are repelled by the electrode and by each other and disperse outwardly into the surrounding air. Ion movement from the electrode to the region of objects that are to be protected is usually accelerated by providing an air flow from the electrode to the object region.
Air ionizing systems intended for static charge suppression are usually designed to generate both positive and negative ions as the charges to be suppressed may be of either polarity. This may be accomplished by using two electrodes having opposite voltages or by periodically reversing the voltage on a single electrode. Production of both types of ion simultaneously tends to reduce the effective range of the apparatus as intermixed positive and negative ions rapidly neutralize each other by charge exchange.
Prior U.S. Pat. No. 4,542,434 of Scott J. S. Gehike et al, issued Sept. 17, 1985 and entitled "Method and Apparatus for Sequenced Bipolar Air Ionization" (assigned to the assignee of the present application) describes a method and apparatus which extends the range of bipolar air ionizers and offers other advantages as well. In the system of that patent, timing signals initiate positive and negative ion generation at spaced apart electrodes during alternate time periods which are separated by off intervals during which no ion generation occurs. This allows an air flow to carry each pulse of ions a substantial distance away from the electrodes before significant intermixing and mutual neutralization of the two types of ions begins.
Precise control of the ion output rate is desirable in apparatus of the above described kind. Effective static charge suppression at a particular location requires that the ratio of positive to negative ions be within a narrow range of values and that the total concentration of ions in the air also be at or close to an optimum value. An excess of ions of one polarity can have the counter-productive effect of imparting charge to objects. A low concentration of ions may not adequately neutralize static charges and an overly high concentration may also have adverse effects. The optimum ratio of positive to negative ions and the optimum total ion concentration that are needed vary from location to location. The optimum ratio and concentration may also vary at a particular location over a period of time because of changes in activities, equipment, air flow patterns or other conditions at the location. The air ion content at the location can also depart from the desired levels because of changes in the ionizing apparatus itself such as electrode deterioration from corrosion, utility power line voltage fluctuations or other causes.
Thus the air ionizing apparatus should enable separate adjustment of the rates of generation of both positive and negative ions and the ion content of the air at the location should be monitored so that readjustments can be made when changed conditions make that advisable.
In the system of the above identified U.S. Pat. No. 4,542,434, positive and negative electrode voltages, the timing of periods of positive and negative ion generation and the duration of the off periods between periods of ion generation can each be independently adjusted. This enables tuning of the system to provide a ratio of positive to negative ions and a total ion concentration that is suited to the needs of the particular location where the system is installed. Ion levels at the site can then be monitored with sensing instruments and manual readjustments can be made when changed conditions make that necessary. It would be advantageous if the monitoring and readjustment process were accomplished automatically and on a continuous basis. The system may then respond more quickly to a sudden change in conditions that calls for a change in the output rate of ions of a particular polarity or of both polarities.
Feedback systems have heretofore been devised for the purpose of automatically adjusting the ion output rate of air ionizers to maintain a desired air ion content under changing conditions. One or more ion sensing devices produce signals indicative of changes in the ion content of the air. A feedback circuit then varies the high voltage on the ionizing electrode in response to changes in the signal to maintain the ion content at the preferred level.
Copending U.S. patent application Ser. No. 085,082 of Arnold J. Steinman et al filed Aug. 11, 1987 and entitled "Self-Regulating Ion Emitter" (assigned to the assignee of the present invention and now issued as U.S. Pat. No. 4,809,127) discloses a feedback system which varies electrode voltage in response to internally sensed variations of ion output rather than in response to an external ion sensor.
The extent to which such feedback systems can compensate for changes in the ion content of the air is dependent on the range of adjustment of electrode voltage that is available. The available range of electrode voltages also limits the speed of response to a sudden depletion of the ion content in the air. An undesirably long time may be needed for restoration of the preferred ionization level.
The range of adjustment of ion output cannot, as a practical matter, be extended simply by providing a high voltage source that can produce higher voltages. Electrode voltages exceeding about 20,000 volts would cause an undesirable generation of ozone and problems with arcing would become severe. Thus air ionizing systems have a maximum operating voltage that is below that level.
The effects of the limited range of voltage adjustment are more pronounced in the case of air ionizers of the above described cyclical type which do not generate ions continuously, but operate instead on a pulsed basis, off periods being alternated with the intervals of ion generation. As the generation of ions of a particular polarity occurs only intermittantly, a longer period of time is needed to correct a sudden depletion of ions of that polarity with the ionizer operating at maximum voltage.
Thus it would be advantaqeous to enable feedback control of pulsed or cyclically operated air ionizers in a manner that is not limited by the above described constraints imposed by the maximum available voltage.
The present invention is directed to overcoming one or more of the problems discussed above.